The long-term goal is to understand the contractile mechanism, the basis of "resting" stretch resistance in smooth muscle, and the control of these functions by neurotransmitters, hormones, ions and pharmacological agents. Smooth muscle fibers in main pulmonary artery, and pulmonary arterioles of mammals, are being studied using electrophysiological, pharmacological, histochemical and ultrastructural methods. Details of the innervation of the muscle fibers, by catecholaminergic, cholinergic, purinergic or peptidergic nerves and the relationship of muscle fibers to one another of the resting membrane and the ionic basis of the resting membrane and the ionic basis of the resting membrane potential are under investigation. Mechanical responses to pharmacological agents in resistance vessels less than 250 microns in diameter can now be studied.